Recent legislation has imposed new constraints on the operation of automotive service stations. In order to prevent the contamination of groundwater supplies, many states are now enacting laws which are designed to deter or prevent leakage from fuel storage tanks and the supply lines interconnecting the storage tanks and the fuel dispensers. These laws have become known as secondary containment laws.
In essence, these laws require that precautions be taken to prevent the occurrence of leakage and the possibility that fuel lines or fuel tanks which have become defective will develop leaks which can continue to allow fuel to seep out into the underground in the vicinity of the service station. The term secondary containment means the provision of a second enclosure surrounding the primary or main fuel enclosure so that if a leak does occur in the primary enclosure it will be confined within the secondary enclosure and cannot seep out.
Such laws impose a significant hardship on existing service stations in which equipment is already in place. To bring such stations into compliance with these laws by installing such a secondary enclosure would require digging up the existing fuel tanks and replacing them with new equipment having the inner and outer containment construction. It would likewise entail providing new plumbing for the supply lines from the fuel storage tank to the pumps in order to bring these lines up to code so that they satisfy the legal requirements.
The intent and purpose of so-called secondary containment laws is to ensure against leakage of fuel and other pollutants and contaminants into the subsoil surrounding a service station location. Utilization of means other than a second container is also permitted and contemplated by such laws.
An alternative approach to secondary containment involves equipment to detect a leak and upon such detection interrupting the operation of fuel supply pumping. A "leak" within the scope of National Fire Protection Association (NFPA) standards involves a very low flow rate; presently, such low flow rate is 0.050 gallons per hour. Accurate detection of such a very low flow rate has presented problems. It should be borne in mind that the dynamic range of flow rate is quite large. Many times more gallons per hour flow during peak flows in a dispensing operation than the very low flow rate involved in such a "leak." Particularly toward the end of a dispensing operation the flow rate of gasoline being pumped is substantially lower than such peak flow rate. It is undesirable not only to fail to detect a leak but also to generate what amounts to a false alarm when there is no leak. In prior art approaches to a system for leak detection and pump interruption, the organization of the system lends itself to an undue number of such false alarms.